6.12.3.3 Halocarbons

In addition to their direct radiative forcing, chlorinated and brominated halocarbons
can lead to a significant indirect forcing through their destruction of stratospheric
O3 (Section 6.4). By destroying stratospheric O3, itself
a greenhouse gas, halocarbons induce a negative indirect forcing that counteracts
some or perhaps all (in certain cases) of their direct forcing. Furthermore,
decreases in stratospheric O3 act to increase the ultraviolet field of the troposphere
and hence can increase OH and deplete those gases destroyed by reaction with
the OH radical (particularly CH4); this provides an additional negative forcing.
Quantifying the magnitude of the negative indirect forcing is quite difficult
for several reasons. As discussed in Section 6.4, the
negative forcing arising from the O3 destruction is highly dependent on the
altitude profile of the O3 loss. The additional radiative effect due to enhanced
tropospheric OH is similarly difficult to quantify (see e.g., WMO, l999). While
recognising these uncertainties, estimates have been made of the net radiative
forcing due to particular halocarbons, which can then be used to determine net
GWPs (including both direct and indirect effects). This was done by Daniel et
al. (1995), where it was shown that if the enhanced tropospheric OH effect were
ignored, and the negative forcing due to O3 loss during the 1980s was -0.08
Wm-2, the net GWPs for the bromocarbons were significantly negative, illustrating
the impact of the negative forcing arising from the bromocarbon-induced ozone
depletion. While the effect on the chlorocarbon GWPs was less pronounced, it
was significant as well. Table 6.10 updates the results
from Daniel et al.’s “constant-alpha” case A as in WMO (l999),
where the effectiveness of bromine for O3 loss relative to chlorine (called
alpha) has been increased from 40 to 60. The updated radiative efficiency of
CO2 has also been included. An uncertainty in the 1980 to 1990 O3 radiative
forcing of -0.03 to -0.15 Wm-2 has been adopted based upon Section
6.4, and these correspond (respectively) to the maximum and minimum GWP
estimates given in Table 6.10.

6.12.3.4 NOx and non-methane hydrocarbons

The short lifetimes and complex non-linear chemistries of NOx and NMHC make
calculation of their indirect GWPs a challenging task subject to very large
uncertainties (see Chapter 4). However, IPCC (l999)
has probed in detail the issue of the relative differences in the impacts of NOx upon O3 depending on where it is emitted (in particular, surface emissions
versus those from aircraft). Higher altitude emissions have greater impacts
both because of longer NOx residence times and more efficient tropospheric O3
production, as well as enhanced radiative forcing sensitivity (see Section
6.5). Two recent two-dimensional model studies (Fuglestvedt et al., l996;
Johnson and Derwent, l996) have presented estimates of the GWPs for NOx emitted
from aircraft. These studies suggest GWPs of the order of 450 for aircraft NOx emissions considering a 100-year time horizon, while those for surface emissions
are likely to be much smaller, of the order of 5. While such numerical values
are subject to very large quantitative uncertainties, they illustrate that the
emissions of NOx from aircraft are characterised by far greater GWPs than those
of surface sources, due mainly to the longer lifetime of the emitted NOx at
higher altitudes.